This invention relates to a blood pump, and more particularly, to a combined pump and motor that is to be disposed in the bloodstream of a patient to pump or assist in the pumping of blood throughout the patient,s circulatory system.
It is desired that a motor/pump of this type have as small a size as possible consistent with the pumping requirements of the device. The suspension of the rotor with respect to the stator is a key to the miniaturization of the system. Where it is possible to minimize the structure by which the rotor is suspended with respect to the stator, it becomes possible to minimize the overall diameter of the motor and pump combination. It is also desired that a motor/pump of this type be constructed without radial seals that can break down and leak. Where it is possible to use a method of suspension of the rotor that operates in blood, the rotary seal can be avoided.
It has been an objective of the present invention to provide an improved rotor suspension system to minimize the size of the motor and pump combination, and to provide suspension without radial seals.
In accordance with the present invention, the stator has a cylindrical internal surface. The rotor has a cylindrical external surface that is substantially coextensive with the stator cylindrical surface. The rotor carries impeller blades to drive blood internally through the motor. There is a gap between the rotor and stator cylindrical surfaces.
During operation, the surfaces of the rotor and stator are so oriented that their relative motion produces a pressure distribution on the rotor that supports the rotor radially. The pressure distribution also creates a pumping action causing blood to flow through the gap between rotor and stator. Blood flow through this gap is further enhanced by the pressure generated axially across the rotor through the action of the impellers. The blood flowing axially through the rotor by the combined pumping action of the hydrodynamic bearing and the pressure generated by the impellers is hereinafter referred to as leakage flow. Since the gap between the rotor and stator is of the order of 0.008" in the radial direction, and since it is this gap with the blood flowing through that provides the suspension for the rotor, it can be seen that the rotor suspension is extremely small in the radial direction. Furthermore, because blood flows directly through the bearing, radial seals do not need to be included.
In the preferred form of the invention, the stator has axially-extending vanes at its inlet and outlet. These vanes have their radial inner edges pressed into the external surface of an elongated hub passing through the center of the rotor. Intermediate stator vanes, axially spaced from the inlet and outlet stator vanes, project radially outwardly from the hub. The rotor has an outside cylindrical surface that cooperates with a motor stator inside cylindrical surface, these surfaces being located between the inlet and outlet vanes. These surfaces create one location wherein the relative motion of mating surfaces provides a hydrodynamic bearing.
The hub has two axially spaced cylindrical surfaces. The rotor has two axially spaced sets of impeller blades that are mounted on the rotor and are terminated internally in a hub support cylinder. The support cylinder of each set of impeller blades cooperates with the hub cylinder to create radially inner gaps with mating surfaces that provide two additional hydrodynamic bearings.
Each of the cylindrical surfaces on the hub are in the form of an annular groove that is a shallow U-shape in longitudinal section. Similarly, the cylindrical surface of the stator is in the form of a shallow U-shaped annular groove in longitudinal section. These shallow U-shaped grooves axially capture the rotor and maintain it centered within the stator. The force of the impeller tends to drive the rotor axially as it rotates in its blood pumping function. That axial force is frictionally resisted by the engagement of radial surfaces between the rotor and the shallow grooves on the stator and hub. Alternatively, thrust-resisting magnets may be mounted in the stator support ring to form a thrust-resisting system.
In a motor pump of this type wherein blood is employed as a hydrodynamic bearing, shear stress should be high enough to provide forces that can prohibit cells from aggregating and thereby leading to thrombus formation. However, it is critically important to assure that shear stress is not so high as to cause blood cell destruction. Furthermore, cell destruction is due not only to shear stress, but also to exposure time of the cells to shear stress; thus, it is important to provide leakage flow high enough to minimize residence time of the cells in the gap. A level of shear stress and exposure time at approximately the threshold level for cell destruction will satisfy both requirements of minimizing thrombus formation and minimizing cell destruction. The parameters that contribute to shear stress are the gap size and the velocity. Residence time in the gap is dependent upon leakage flow, which in turn is dependent upon gap size. The larger the gap size, the lower is the shear stress and the higher is the leakage flow. In order to minimize cell damage in the gap, shear stress should be maintained below 2500 dynes per square centimeter and residence time of cells in the gap should be below 0.1 second. However, the gap size must be kept low to minimize violent eccentric motion of the rotor with respect to the stator.
Blood cell destruction within the main flow path of the pump, through the impeller and stator vanes, is also dependent upon shear stress. However, since the shear stress in the main flow path is not readily known, and since the velocity of a cell with respect to the pump surfaces affects shear stress, it is sufficient for the main flow path to minimize velocity as a means of minimizing cell destruction. Velocity should be below 1000 centimeters per second and preferably below 500 centimeters per second.